W . Ujpham — Diversity of the Glacial Drift. 365 



That the later part of the loess deposition was contempo- 

 raneous with the formation of the prominent Altamont mo- 

 raine of the later drift, I ascertained in northwestern Iowa, 

 where this moraine along a distance of 75 miles, from Guthrie 

 County northwestward to Storm Lake, is bordered on its west 

 side by an expanse of loess as high as the crests of the mo- 

 rainic hills, while its elevation above the expanse of till east- 

 ward is from 50 to 75 feet. During the time of deposition of 

 this part of the loess the ice-sheet reached to the Altamont 

 moraine and was a barrier preventing the waters by which the 

 loess was brought from flowing over the lower area of till that 

 reaches thence east to the Des Moines river.* 



These observations in three widely separated regions prove 

 that the loess, like the coarser portions of the modified drift 

 forming sand and gravel plains, was in progress of deposition 

 upon successive areas as fast as the ice-sheet supplying these 

 stratified drift beds receded. Immediately after the land was 

 bared by the retreat of the ice, and even while the ice itself 

 occupied the adjoining land, the loess was being laid down, 

 contemporaneous successively with the earliest till on the 

 southern limit of the drift, with the till of intermediate age 

 in northeastern Iowa, and with the later till enclosed by the 

 Altamont moraine. 



Such being the well demonstrated origin of the loess and its 

 relations with the earlier and later glacial drift, it seems im- 

 possible that the rock gorges described by Mr. Oscar H. 

 Hershey upon the area of the early drift in northwestern 

 Illinois can have been eroded, as he supposes, between the 

 times of deposition of the drift and of the loess.f Instead, I 

 think that the early drift there and its closely ensuing loess so 

 filled the valleys and raised the streams above the beds of the 

 deep preglacial channels that in the places noted by Mr. 

 Hershey the streams were turned aside into preglacial courses 

 of small tributaries or across cols of plateau tracts which had 

 become isolated alongside the valleys by the processes of ordi- 

 nary land erosion and general weathering with rain, rill, and 

 stream sculpture. This view brings harmony with Prof. Salis- 

 bury's assignment of the loess upon the early drift region to a 

 time contemporaneous with the retirement of the ice-sheet 

 from its farthest boundaries. 



* Geol. and Nat. Hist. Survey of Minnesota, Ninth An. Rep. for 1880, pp. 

 307-31-1, 338. 



f Am. Geologist, vol. xii, pp. 314-323, Nov., 1893. 



